Multi-mode air drying system

ABSTRACT

A compressed air drying system is provided for removing moisture from compressed air. The dryer operates in two modes in response to the demand for compressed air. In a first mode of operation, a refrigeration compressor runs continuously and the speed of a condenser fan is varied to maintain a constant cooling temperature. In a second mode of operation, the refrigeration compressor runs intermittently between on and off periods. As result, the cooling temperature fluctuates during the second mode of operation.

BACKGROUND

The present inventions relate generally to industrial air dryers for compressed air systems.

Compressed air is commonly used in factories to power pneumatic tools and to blow air onto various surfaces for cleaning, expanding bags, moving parts, etc. Typically, factories have a centralized compressed air system installed that feeds a network of compressed air piping that supplies numerous tools or stations with compressed air. Thus, one or more centralized air compressors may be used to supply an entire factory space with compressed air.

However, it is known that air compressors which draw air from the surrounding atmosphere also introduce moisture into the compressed air from the water vapor naturally contained in atmospheric air. Moisture within compressed air used in factories can cause numerous problems. For example, in the case of power tools that use compressed air as a power source, moisture within the supplied compressed air can cause corrosion of the internal components of the tool. In addition, where compressed air is blown onto surfaces, any moisture within the compressed air will also be blown onto the surface along with the blown air. This can be particularly problematic where it is a requirement that the surface remain dry, such as food packaging operations, and can also be a problem with delicate surfaces that might be damaged by water particles within the compressed air.

Due to the problems associated with moisture within compressed air systems, various types of air drying systems may be used in industrial factories to remove moisture contained within compressed air. While such systems are useful and adequately address the potential problems associated with moisture in compressed air, such systems can be expensive to operate and maintain. Thus, it would be desirable to provide improved air drying systems for industrial factories.

SUMMARY

A drying system is described for removing moisture from compressed air. The system includes a refrigeration system that operates in two modes depending on the compressed air demand. When compressed air demand is high or moderate, the refrigeration compressor runs continuously and the speed of the condenser fan is varied to maintain a constant cooling temperature. When compressed air demand is low, the compressor runs intermittently between on and off periods to save energy. The invention may also include any other aspect described below in the written description or in the attached drawings and any combinations thereof.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The invention may be more fully understood by reading the following description in conjunction with the drawings, in which:

FIG. 1 is a schematic of an air dryer;

FIG. 2 is a chart of temperature variation during a second mode of operation; and

FIG. 3 is a flow chart of a method of operating an air dryer.

DETAILED DESCRIPTION

Referring now to the figures, and particularly FIG. 1, a schematic for an industrial air dryer 10 for an industrial factory is shown. Although not illustrated, it is understood that the compressed air inlet 12 is connected to a compressed air supply which typically includes one or more air compressors that draw air from the surrounding atmosphere and compresses it to a pressure between 100-200 psi. The compressed air outlet 14 is connected to a network of tools or stations in the factory that use the compressed air for a variety of uses. It may also be desirable for the compressed air system to also have various compressed air storage tanks between the compressor(s) and the air dryer 10 and/or between the air dryer 10 and the tools and/or stations where the compressed air is used. It is further understood that a controller is provided either on the air dryer 10 or located remotely to control the air dryer 10 as described further below.

As shown in FIG. 1, compressed air from the air inlet 12 preferably enters the precooler side 16A of a precooler/preheater 16. The air then exits the precooler/preheater 16 and enters a main cooler 18. After exiting the main cooler 18, the air enters a moisture separator 20. The air then reenters the precooler/preheater 16 on the preheater side 16B, and thereafter, exits the air dryer 10 through the air outlet 14.

The precooler/preheater 16 is a heat exchanger 16 that exchanges heat between the incoming air flow and the outgoing air flow. That is, the incoming air flow is warm relative to the outgoing air flow. As described below, the air is cooled within the dryer 10 to withdraw moisture from the air. Thus, the precooler/preheater 16 increases efficiency by cooling the incoming air with the outgoing air prior to additional cooling that occurs thereafter. Also, it is undesirable for the outgoing air to be too cool since this would cool the compressed air piping and cause condensation of water vapor on the exterior of the piping. Thus, the precooler/preheater 16 prevents this from happening by heating the outgoing air using the warm incoming air.

The main cooler 18 is another heat exchanger 18 that performs the primary cooling of the compressed air. As described further below, the main cooler 18 may use a refrigerant cooling system 22 to cool the compressed air. After the compressed air has been cooled by the main cooler 18 (e.g., to below 5° C.), the moisture separator 20 withdraws moisture from the compressed air. The withdrawn moisture is then removed through a drain 24. Thus, the compressed air entering the preheater side 16B of the precooler/preheater 16 and exiting the dryer 10 has been dried by removing water vapor from the compressed air. It is understood that airflow through the air dryer 10 need not be separately forced or circulated therethrough, but instead may flow through the dryer 10 as air is used by the compressed air demand and replaced by the compressed air supply. That is, any compressed air that flows to the compressed air demand from the compressed air supply must first pass through the dryer 10 (or another dryer in the system) due to the location of the dryer 10 between the supply and demand.

Preferably, the main cooler 18 is cooled with a refrigerant based cooling system 22. Thus, the refrigerant side 18B of the main cooler 18 may be considered to be an evaporator 18B where the refrigerant evaporates and absorbs heat from the compressed air side 18A of the main cooler 18. The refrigerant vapor is then compressed to a higher pressure (and higher temperature) by a refrigerant compressor 26. The refrigerant then passes through a refrigerant condenser 28 that cools and liquefies the refrigerant. The condenser 28 is another heat exchanger 28 with a fan 30 that blows ambient air across the condenser 28 to dissipate heat from the refrigerant. A filter/dryer 32 may be provided thereafter to clean the refrigerant. An expansion valve 34 then converts the refrigerant back into a vapor for reentry into the refrigerant side 18B of the main cooler 18.

In order to improve efficiency of the dryer 10, two modes of operation are provided depending on the demand for compressed air. That is, when the demand for compressed air is above a threshold (e.g., when a factory is running at full capacity and many air tools are being used), the dryer 10 operates in a first mode of operation. However, when the demand for compressed air is below a threshold (e.g., only partial factory utilization), the dryer 10 shifts into a second mode of operation. The first and second modes of operation are principally distinguished from each other by the operation of the refrigerant compressor 26 and the condenser fan 30 in the two modes. That is, in the first mode, the compressor 26 runs continuously and the speed of the condenser fan 30 is varied to adjust the refrigeration system 22 to the compressed air demand. By contrast, in the second mode, the compressor 26 runs intermittently between on periods 36 when the compressor 26 is running and off periods 38 when the compressor 26 is turned off and not running. Thus, during the on periods 36 when the compressor 26 is running, refrigerant flows through the main cooler 18 like in the first mode. However, during the off periods 38, the refrigerant does not flow through the main cooler 18 since the compressor 26 is not operating. Thus, the compressed air continues to flow through the main cooler 18 during the off periods 38 but the refrigerant is not withdrawing heat from the main cooler 18.

The described method is particularly useful with a fixed speed refrigerant compressor 26, which is relatively inexpensive and reliable but draws a constant amount of power even when the cooling requirement is minimal. It is envisioned that the on and off periods 36, 38 will each be typically between 2 and 30 minutes. Off periods 38 less than 2 minutes would indicate that the cooling requirement is relatively high and less efficiency gains are likely to be achieved with short on and off periods 36, 38. On the other hand, off periods 38 greater than 30 minutes would effectively stop most cooling of the compressed air since the main cooler 18 will typically warm to the temperature of the compressed air in a long off period 38.

Various types of thresholds for controlling the shift between the first and second modes may be used. Various measurements may also be used for determining compressed air demand and controlling the condenser fan 30 in response thereto. In the preferred embodiment, the temperature T_(i) of the refrigerant at the inlet of the main cooler 18 between the expansion valve 34 and the main cooler 18 may be used both as a control for the condenser fan 30 in the first mode and as a threshold between the first and second modes. For example, in the first mode, the fan 30 may be varied to maintain a constant refrigerant temperature T_(i) by increasing the fan 30 speed to decrease the refrigerant temperature T_(i) and decreasing the fan 30 speed to increase the refrigerant temperature T_(i). The refrigerant temperature T_(i) may also be used indirectly as a threshold for the first and second modes by shifting to the second mode when the fan 30 speed drops below a set value.

The refrigerant temperature T_(i) may also be used to determine the on periods 36 and off periods 38 in the second mode. As shown in FIG. 2, the compressor 26 is turned on 36 in the second mode after a temperature of the main cooler 18 (e.g., T_(i)) rises 40 to a set value. The compressor 26 then cools the main cooler 18 while the compressor 26 is running. Once the temperature of the main cooler 18 drops 42 to another set value, the compressor 26 turns off 38. During this time, the compressed air is still flowing through the main cooler 18 and is being cooled by the cold energy stored in the main cooler 18. That is, the main cooler 18 and stagnant refrigerant therein act as a heat sink that stores the cold temperature generated during the on period 36. However, since the refrigerant is not being cooled during the off period 38 and is not flowing through the main cooler 18, the flowing compressed air eventually absorbs the stored cold energy and causes the temperature of the main cooler 18 to rise 40. This intermittent sequence is repeated during the second mode so that the compressor 26 need not be running constantly and energy can be saved during off periods 38.

As shown in FIG. 2, the temperature of the main cooler 18 (e.g., T_(i)) preferably fluctuates below and above the freezing point of water during the second mode. By contrast, the main cooler 18 temperature is preferably held constant during the first mode at a temperature slightly above 0° C. (e.g., 1-6° C.). Normally, it would be undesirable to allow the temperature of the main cooler 18 to drop below the freezing temperature, since this will allow the moisture in the compressed air to freeze and collect within the main cooler 18. However, in the described second mode, allowing a temperature drop below freezing causes the main cooler 18 to act as a heat sink to store cold energy. Thus, frosting that may occur within the main cooler 18 due to moisture in the compressed air acts temporarily as a heat sink but melts and dissipates during the off period 38 as the temperature rises 40 above freezing. In order to improve efficiency in the second mode during the on periods 36, it may be preferable to run the condenser fan 30 at full speed to apply maximum cooling to the main cooler 18. The condenser fan 30 may also be run at a constant speed during the on periods 36 as opposed to a varying speed as in the first mode.

As previously noted, it is also possible to use other values or temperatures to control the fan 30 speed, the shift between the first and second modes, and the on and off periods 36, 38 in the second mode. For instance, a temperature difference may also be used. For example, the temperature T_(o) of the refrigerant may also be measured at the outlet of the main cooler 18 between the main cooler 18 and the compressor 26. A large temperature difference between T_(i) and T_(o) generally indicates that there is a high compressed air demand since a higher level of heat is being absorbed from the refrigerant in the main cooler 18. Thus, the difference between T_(i) and T_(o) may be used in varying the fan 30 speed in the first mode, determining when to shift to the second mode, and determining the on and off periods 36, 38.

Turning to FIG. 3, a flow chart of an embodiment of the invention is illustrated. The flowchart begins with the dryer 10 operating in the first mode by measuring the temperature of the dryer 10 (e.g., T_(i)) (44). The speed of the condenser fan 30 is then varied based on the measured temperature in order to maintain the temperature at a constant level (46). It is desirable to maintain the temperature of the main cooler above 0° C. in the first mode (e.g., 1-6° C.). The speed of the fan 30 is then checked (48), and if the fan 30 speed is above the threshold, the dryer 10 stays in the first mode and continues to vary the condenser fan 30 speed as needed to maintain a constant temperature (44-48). However, if the speed of the fan 30 drops below a threshold, the dryer 10 switches into the second mode of operation. It may be desirable for the fan 30 to drop below a threshold for a set amount of time before the dryer 10 switches into the second mode.

In the second mode of operation, the speed of the fan 30 is preferably set to a constant speed that is an increase over the fan 30 speed before the shift into the second mode (50). For example, the fan 30 speed may be increased to full speed. Typically, the second mode will begin with an on period 36 where the compressor 26 continues to run. The temperature of the main cooler 18 is then measured again (52), and the dryer 10 remains in the on period 36 as long as the temperature remains above a set value (52-54). For example, in FIG. 3, the set value is 0° C. (54). However, as shown in FIG. 2, the set value may be about −5° C. Preferably, the main cooler 18 does not cool to less than −10° C. during the on period 36. When the temperature drops 42 to the set value (preferably at least 0° C.), the compressor 26 is turned off and the fan 30 may also be turned off (56). It may be desirable for the temperature to remain below the set value for a set amount of time before the compressor 26 and fan 30 are turned off. The temperature is then measured again (58) and the dryer 10 remains in the off period 38 as long as the temperature remains below a set value (58-60). For example, in FIG. 3, the set value is 0° C. (60). However, as shown in FIG. 2, the set value may be about 2° C. Preferably, the main cooler 18 does not warm to more than 6° C. during the off period 38. When the temperature rises 40 to the set value (at least 0° C.), the compressor 26 and the fan 30 are turned back on (62). The amount of time of the off period 38 is also determined (64), and if the time of the off period 38 was less than a set value, the dryer 10 then shifts back into the first mode (44). Otherwise, the dryer 10 stays in the second mode (52) where the compressor 26 runs intermittently between on and off periods 36, 38.

While preferred embodiments of the inventions have been described, it should be understood that the inventions are not so limited, and modifications may be made without departing from the inventions herein. While each embodiment described herein may refer only to certain features and may not specifically refer to every feature described with respect to other embodiments, it should be recognized that the features described herein are interchangeable unless described otherwise, even where no reference is made to a specific feature. It should also be understood that the advantages described above are not necessarily the only advantages of the inventions, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the inventions. The scope of the inventions is defined by the appended claims, and all devices and methods that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein. 

The invention claimed is:
 1. A method of drying compressed air, comprising: supplying compressed air to one side of a first heat exchanger; supplying refrigerant to another side of the first heat exchanger; in a first mode when a demand for the compressed air is above a threshold, running a compressor continuously and varying a speed of a condenser fan in response to the demand for the compressed air, the compressor compressing the refrigerant after passing through the first heat exchanger and the condenser fan blowing air across a second heat exchanger to cool and condense the refrigerant after passing through the compressor; and in a second mode when a demand for the compressed air is below the threshold, running the compressor intermittently between on periods and off periods, the compressed air passing through the first heat exchanger while the refrigerant passes through the first heat exchanger during the on periods, and the compressed air passing through the first heat exchanger without the refrigerant passing through the first heat exchanger during the off periods, wherein in the second mode, the speed of the condenser fan is set to a constant higher speed than the speed of the condenser fan prior to a shift from the first mode to the second mode to maintain a threshold temperature on the one side of the first heat exchanger during the off periods.
 2. The method according to claim 1, wherein each of the off periods is between 2 and 30 minutes.
 3. The method according to claim 1, wherein each of the on periods is between 2 and 30 minutes.
 4. The method according to claim 1, wherein the compressor is a fixed speed compressor.
 5. The method according to claim 1, further comprising preheating and precooling the compressed air in a third heat exchanger, the compressed air passing through one side of the third heat exchanger before passing through the first heat exchanger, and the compressed air passing through another side of the third heat exchanger after passing through the first heat exchanger.
 6. The method according to claim 1, further comprising separating moisture from the compressed air after the compressed air passes through the first heat exchanger.
 7. The method according to claim 1, further comprising converting the refrigerant from a liquid to a gas before the refrigerant passes through the first heat exchanger.
 8. The method according to claim 1, further comprising measuring a first temperature of the refrigerant and determining the on periods and off periods in response to the first temperature.
 9. The method according to claim 8, wherein the first temperature is measured between an expansion valve and the first heat exchanger.
 10. The method according to claim 9, further comprising measuring a second temperature of the refrigerant between the first heat exchanger and the compressor, and determining the on periods and off periods in response to a difference between the first temperature and the second temperature.
 11. The method according to claim 1, wherein the refrigerant passing through the first heat exchanger is cooled below 0° C. during the on periods and the refrigerant within the first heat exchanger warms above 0° C. during the off periods.
 12. The method according to claim 11, wherein the refrigerant passing through the first heat exchanger is cooled to not less than 10° C. during the on periods.
 13. The method according to claim 11, wherein the refrigerant within the first heat exchanger warms to not more than 6° C. during the off periods.
 14. The method according to claim 1, wherein the speed of the condenser fan is varied in the first mode to maintain a constant temperature of the refrigerant such that the speed of the condenser fan is increased to decrease a temperature of the refrigerant and the speed of the condenser fan is decreased to increase the temperature of the refrigerant, the threshold being a speed of the condenser fan in the first mode below a set value.
 15. The method according to claim 1, wherein the condenser fan runs at a constant speed during the on periods.
 16. The method according to claim 1, wherein the condenser fan runs at full speed during the on periods.
 17. The method according to claim 1, wherein each of the off periods is between 2 and 30 minutes, and each of the on periods is between 2 and 30 minutes.
 18. The method according to claim 17, further comprising preheating and precooling the compressed air in a third heat exchanger, the compressed air passing through one side of the third heat exchanger before passing through the first heat exchanger, and the compressed air passing through another side of the third heat exchanger after passing through the first heat exchanger, further comprising separating moisture from the compressed air after the compressed air passes through the first heat exchanger, and further comprising converting the refrigerant from a liquid to a gas before the refrigerant passes through the first heat exchanger.
 19. The method according to claim 18, wherein the compressor is a fixed speed compressor, further comprising measuring a first temperature of the refrigerant and determining the on periods and off periods in response to the first temperature, wherein the refrigerant passing through the first heat exchanger is cooled below 0° C. during the on periods and the refrigerant within the first heat exchanger warms above 0° C. during the off periods.
 20. The method according to claim 19, wherein the refrigerant passing through the first heat exchanger is cooled to not less than 10° C. during the on periods, the refrigerant within the first heat exchanger warms to not more than 6° C. during the off periods, and the condenser fan runs at a constant speed during the on periods. 